1. Field of the Invention
This invention relates generally to a system for electrostatically printing an image and more particularly concerns a method of liquid ink development.
2. Description of Related Art
Many electrostatic developing systems use dry particle toners to create toned images on imaging drums. However, dry particle toners have numerous disadvantages. Because small dry toner particles become readily airborne, causing health hazards and machine maintainability problems, their diameters are seldom less than 3 microns, which limits the resolution obtainable with dry toner particles. Further, thick layers of dry toner, such as is necessary in color images, causes significant paper curl and thereby limits duplex applications. Therefore, there has been a great desire to develop liquid development systems.
Liquid ink development systems are generally capable of very high image resolution because the toner particles can safely be ten or more times smaller than dry toner particles. Liquid ink development systems show impressive grey scale image density response to variations in image charge and achieve high levels of image density using small amounts of liquid developer. Additionally, the systems are usually inexpensive to manufacture and are very reliable. However, liquid ink development systems are based on volatile liquid carriers and, as a result, they pollute the environment. Consumers are often wary about using such liquid development systems for fear of health hazards. Therefore, there is a strong desire for a liquid ink development system that does not create airborne pollution.
Prior art liquid ink development systems operate such that the photoconductor surface rotates through the developer bath to make contact with the toner. In these systems, the toner particles are attracted to the latent electrostatic image on the photoconductor surface. The motion of the toner particles in the imagewise electric field is generally called electrophophoresis and is well known in the art. However, the liquid carrier also wets the photoconductor surface. It is very difficult to transfer the toner image to paper without either first removing the liquid carrier from the photoconductor surface or using the liquid carrier to enable transfer to the paper and subsequently removing the liquid carrier from the paper, In both cases, the liquid carrier must be removed by processes that must include evaporation of the liquid carrier into the air, which causes airborne pollution.
U.S. Pat. No. 4,306,009 to Veillette et al. discloses a vinyl polymeric gel (called a "gelatex") used in a developer as a fixative and as a dispersant. The gelatex component is present in the carrier as a stable dispersion and is substantially depleted as multiple copies are produced. The disclosed gelatex is not in any sense used as a transfer layer as described below.